The long term goal is to understand the molecular mechanism of hepatocarcinogenesis so that novel strategies for the effective prevention and treatment of liver cancer can be developed. Epigenetic abnormity is a major hallmark in hepatocellular carcinogenesis. Loss of the tumor suppressor phosphatase and tensin homolog (PTEN) in hepatocytes causes progressive steatohepatitis and liver cancer;the epigenetic silencing of PTEN and other tumor suppressors is associated with dimethylation of histone H3 at lysine-9 (H3K9me2), a key suppressive mark for gene silencing. The histone methyltransferase G9a plays a key role in embryonic development through catalyzing H3K9me2 and facilitating DNA methylation by DNA methyltransferases. G9a mediates gene repression by a group of transcriptional repressors/corepressors important in steatohepatitis and carcinogenesis. Knockdown of G9a causes dramatic down-regulation of telomere reverse transcriptase and apoptosis of cancer cells. G9a is induced in human liver cancer;however, little is known about the in vivo importance of G9a in liver pathophysiology and carcinogenesis, largely due to the embryonic lethality of G9a-null mice. The objective of this proposal is to elucidate the pathophysiological importance of G9a in liver utilizing novel models of mice with hepatocyte-specific knockout (HKO) of G9a and PTEN. Our preliminary data demonstrate that in mouse and human hepatoma cells, a specific G9a inhibitor induces tumor suppressor genes, inhibits cell proliferation, and promotes apoptosis. In contrast, G9a-HKO mice appear normal, suggesting that loss of G9a in adult liver is well-tolerated. The central hypothesis is that G9a plays a key role in steatohepatitis and liver carcinogenesis via promoting lipogenesis and inhibiting tumor suppressor genes, and thus loss of G9a will inhibit hepatosteatosis and cell proliferation, whereas promote the apoptosis of liver cancer cells. This central hypothesis will be tested in two specific aims. Aim 1 will determine the importance of G9a in liver regeneration using models of 2/3 partial hepatectomy in G9a-HKO mice. The working hypothesis is that G9a-HKO mice will have relatively normal liver regeneration despite a moderate/minor delay in the regenerative process. Aim 2 will determine the importance of G9a in steatohepatitis and liver carcinogenesis using mice with hepatocyte-specific double knockout of PTEN and G9a. The working hypothesis is that loss of G9a will prevent PTEN-null-induced progression of hepatosteatosis-steatohepatitis to cirrhosis and liver cancer via inhibiting lipogenesis and cell proliferation whereas enhancing apoptosis. Results from this study will provide crucial novel knowledge about the importance of G9a in normal liver function, liver regeneration, and liver carcinogenesis, which may ultimately help to identify G9a inhibitor as a novel epigenetic drug for the prevention and treatment of liver cancer, a deadly disease with increasing incidence in the USA.